Refrigerate the peanut butter cups for about 10 minutes to make them easier to chop. You can also use mini peanut butter cups which don’t need to be chopped first (Trader Joe’s makes mini peanut butter cups in both dark and milk chocolate - these are great in the recipe!).
I have not tested these bars with natural peanut butter. I use Jif Creamy Peanut Butter to make this recipe.